This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We propose to use solution, small-angle x-ray scattering to study the structure and conformational dynamics of two viral proteins: influenza hemagglutinin and the hepatitis B virus capsid protein. The studies will focus on generating moderateresolution 3D reconstructions as well as quantitative analysis of time-resolved structural changes. Enveloped viruses such as influenza virus use specialized protein machinery to fuse their membrane with the cellular membrane of host cells and deliver their genome for replication. In influenza virus, the trimeric hemagglutinin (HA) glycoprotein is responsible for both host cell attachment and membrane fusion. A mechanistic understanding of biological machines such as HA requires both detailed structures and an understanding of the dynamics and energetics that govern their function. The states that actively drive the fusion process have proven to be refractory to classical structure determination. SAXS will be used to bridge the significant gaps in our structural understanding of HA and to characterize the mechanism of HA-mediated membrane fusion using time-resolved studies. Our proposed studies of hepatitis B virus (HBV) capsids focus on the assembly and drug-induced misassembly processes. HBV is one of the few systems for which interactions between virus and drugs targeted against its protective capsid have been well characterized both structurally and biochemically. It is thus an ideal system for exploring the relation of virus structure and function. Time-resolved studies of HBV capsid assembly reactions will be carried out to identify nucleation states and to monitor the assembly reaction of HBV capsids. Next, the modulation of HBV capsid assembly by antiviral compounds will be investigated.